The invention relates to a device and method for handling freight containers. In particular it is a device and method for measuring the position of a ground slot on which a container may be placed.
A vast amount of freight is shipped in standard shipping containers. At each point of transfer from one transport means to another, for example in ports and harbours, there is a tremendous number of containers that must be unloaded, transferred to a temporary stack, and later loaded on to another ship or to another form of transport.
In a place for handling containers, such as a container yard, containers are usually stacked in substantially rectangular groups of stacks. Containers are usually arranged with long axes ordered parallel in substantially parallel rows. The rows are laid out to provide a clearance between containers in each row, and with a clearance between each row of containers. Space in such container yards is at a premium and so the clearance between container rows and containers is required to be minimal. The high running cost of ships requires that containers be moved between ship and stack as rapidly as possible so that a ship may be unloaded, loaded and turned around in the shortest possible time. To achieve the shortest unloading and loading times container handling equipment has to be partly and/or preferably completely automated in normal operation.
The technical demands of handling containers are great. The tare weight of containers is usually consistent, but the gross weight varies considerably. The width of shipping containers is standardised at 8 ft, but the height varies between from 8 and 9.5 ft. The most common standard lengths are 20 ft and 40 ft long. The 40 ft container is very common today and even longer containers up to 53 ft long are also in use. Thus the size of containers varies as well as the weight. With the increase in average length of containers handled it becomes more difficult and more important to place containers accurately in a specified place, known as a ground slot, in the container yard.
A retrieval problem may occur when a stack of containers is placed so that it leans towards or over a lower stack of containers. A horizontal clearance is required for vertical access between two rows of containers. If the horizontal clearance between rows is insufficient, it may not be possible for an automatic lifting device either to sense or to access a container at the top of a stack.
A container may be handled by a crane, a crane moving on rails, a self-propelled container handling apparatus, or a lift or winch of any type all of which are referred to herein as a crane. Each crane has a lifting device usually incorporating a spreader of some kind that directly contacts a container, to grip it, lift it, lower it and release it. In this description the term spreader is used to denote a part of a lifting device that is in direct contact with a container. Spreaders are normally designed to handle more than one size of container, typically 20-40 ft or 20-40-45 ft long containers.
Automatic container handling apparatuses and systems are used to speed up the process of loading and unloading. An example of a mobile system is known from U.S. Pat. No. 5,780,826. A self-propelled container handling apparatus is disclosed which senses an identification code on a container and places the container in a container yard. The self-propelled handling apparatus uses a vertical lifting sensing means to sense vertical distance, and uses two measuring means for sensing horizontal distances. First, a distance measuring means relating the position of the lifting part of the apparatus to the position of the rest of the apparatus and secondly, ground position satellite (GPS) sensing to track the ground position of the handling apparatus itself. However, it is difficult to land containers rapidly and automatically with equipment of this type. It is also difficult to place containers accurately on the ground using a GPS satellite system as the resolution of such a system is usually more suitable for measuring distances of several meters rather than centimeters.
In a container yard each location for a container placed on the ground, a ground slot, is typically defined by lines painted on the ground. The lines painted on the ground are intended to show the position of the size of container for which the ground slot is laid out. If the support surface is designed to support stacks of 20 ft containers then the painted lines usually indicate 20 ft ground slots. It is normally permissible to place a 40 ft container covering two 20 ft ground slots. There may also be ground slots which are laid out to be used by either one 45 ft or one 40 ft or two 20 ft containers.
If the first container placed on the ground slot is not accurately placed, there may not be sufficient clearance to handle containers in adjacent ground slots efficiently and without causing damage to containers. The amount of time lost and the risk of damage to containers increases if additional containers are subsequently stacked on top of a container that has been misplaced on a ground slot.
In addition to horizontal errors of placement in a direction parallel with or perpendicular to the line of the row and the long axis of a container, a container may be skewed. A skew is defined here as an angular displacement of the long axis of the container with respect to the line of the row.
EP 0302 569 describes a storage and trans-shipment system for containers. The system includes a grid of ground positions laid out as a rectangular grid of wires buried in the surface of a yard. The system described includes a method for an independent container carrying vehicle to navigate to a selected ground position, but does not disclose how a position of a container relative to the ground position may be accurately determined.
GB 1 449 488 describes a storage with automatic handling means. The storage yard includes position indicating elements disposed relative to a position of railway cars on a track. Other position indicating elements are placed to indicate a centre point of a ground position. The devices and methods described do not disclose how to determine if a container is accurately aligned relative to a ground position or parallel to an axis of the ground position.
It is an object of the invention to provide a device and method to measure the horizontal distance between a travelling part of a crane and a ground slot on the ground or on a deck. It is another object of the invention to provide a means and method to measure the horizontal distance between the travelling part of a crane and a ground slot on the ground regardless of length of container. It is another object of the invention to provide a means and method to measure the horizontal distance between the travelling part of a crane and a ground slot on the ground regardless of the direction of approach by the crane with respect to the ground slot. It is a further object of the invention to measure a skew angle between the travelling part of the crane and the ground slot.
These and other objects are realised by a device and method according to the invention. The present invention is a horizontal reference marker placed on the ground arranged relative to, and at a predetermined distance from, the position of a ground slot for a container and a method for measuring the horizontal distance between a travelling part of a crane and the ground slot on the ground for a container. Using known measuring means such as laser arrangements suitably mounted on a travelling part of a crane, the exact location of a ground slot on a horizontal plane can be determined using the present invention.
The main advantage of the present invention is that a container may be lowered accurately onto a ground slot position on the ground or on any other support surface such as a deck or a floor. A ground slot on the ground is used here to describe a stacking target on any substantially horizontal surface such as the ground, a support surface constructed on the ground, a ships deck, a deck or floor inside a building or the loading surface of a vehicle.
The first benefit of the present invention is that a first container accurately placed on the ground means that there would be sufficient clearance between the first container and adjacent containers. This means that the first container, and containers subsequently stacked on top of it may, in the vicinity of containers in adjacent ground slots and stacks, be handled efficiently and without damage to containers.
Another advantage is that a container may be automatically lowered into a precisely measured target position. This offers the important economic benefit of rapid handling of containers, which is extremely important in maintaining low and competitive freight costs. Another advantage with an economic benefit is that clearances between stacks and rows may be kept to a minimum, reducing the area of ground needed for storing containers. A technical advantage of this present invention is that the horizontal position of the container in x, y directions is secured by ground reference marker to a high degree of accuracy. A preferred embodiment includes a method in which skew errors in a horizontal plane are easily measured.
A technical and economic advantage of the device and method is that it may also be used in adverse weather conditions. Despite reduced visibility that would delay or prevent a crane driver from seeing a target position caused by bad weather or poor lighting conditions, for example, the present invention enables the ground position to be accurately determined. The present invention is not limited by direction of approach of a crane towards a ground slot, or by containers of differing lengths. The present invention is very useful in facilitating trade by making container handling fast and keeping costs low. It is also not restricted to shipping ports and may be applied to handling any containerised freight, such as air freight, and containers on and off trains or trucks.